The present invention relates to a battery, and more particularly to a battery having a reinforced structure for preventing the case of a secondary battery from expanding due to an increase of internal pressure.
In a conventional secondary battery which is repeatedly charged and discharged, the internal pressure is increased due to gas generated during charging. When the internal pressure of the battery is increased to the critical pressure or higher, the lower end of the battery case is expanded outward.
FIG. 1 is a partial sectional view of a conventional battery having the lower end expanded due to the internal pressure. In the battery case, positive and negative plates are alternately arranged with a separator interposed therebetween. Typically, such a separator is wound between the positive and negative plates in the battery case.
As shown in FIG. 1, the lower insulation ring of the battery and the lower end of the battery case are expanded and transformed due to the internal pressure. This is because the internal pressure of the secondary battery is increased by gas generated during charging. In the case of a nickel-cadmium battery, oxygen gas generated during charging is absorbed, as expressed by the formula: 2Cd+O.sub.2 +2H.sub.2 O.fwdarw.2Cd(OH).sub.2. However, the rate of absorption of the oxygen (O.sub.2) is different from the rate of generation of the oxygen (O.sub.2), which causes the internal pressure. In other words, as soon as the oxygen is generated, it is impossible to instantly perform the reaction of absorbing the oxygen, although the internal pressure is increased due to generation of oxygen, the absorption speed is increased.
As the battery is made smaller in size, lighter in weight, and higher in performance, the active materials in the battery comprise a large fraction of the total battery volume and the thickness of the battery case must be decreased. However, as the battery case becomes thinner, the lower end of the battery case more severely expands outward, which is an important consideration limiting the design of high-performance battery structures.
The expansion of the lower end of the battery should be considered in fabricating a package cell battery as well as a unit cell battery.
In a cylindrical battery case of one body type, the battery case can be manufactured to have a thick bottom and thin sides. Here, treatment stress increasingly generated during manufacturing increases the strength of the material. However, crack or bursting due to the internal pressure may occur when heat treatment for removing the stress is not performed. In the case of performing the heat treatment for removing stress after a process of shaping, the lower end of the battery case which is a little thickly made of the same material as that of the battery case may be also transformed due to the internal pressure. This is because the material is selected with reference to treatment of the battery case.